This invention relates to a volatile gas detector, particularly to a continuously operated photo-ionization detector (PID), and a method for real-time self-cleaning of the volatile gas detector.
Photo-ionization detectors (PIDs) can detect volatile gases. FIG. 1 shows a conventional portable PID 10 that includes an ultraviolet (UV) lamp 12 and an ionization chamber 14. UV lamp 12 produces UV light including UV photons having energy up to 8.4 electron volts (eV) or more. The UV photons pass through an optical window 16 into ionization chamber 14. In ionization chamber 14, the UV photons collide with and ionize volatile gas molecules having ionization potentials below the energy of the photons, creating ions and electrons.
PID 10 further includes an ion detector 18 having a pair of electrodes 20 and 22, which are typically made of a metal. Ion detector 18 has a high voltage (150 V or more) applied across electrodes 20 and 22 to generate an electrical field. In particular, electrode 22 is biased to a high voltage to attract negatively charged particles (electrons) and repel positively charged particles (ions), and electrode 20 is grounded to collect the positively charged particles (ions). The movement of the ions to electrode 22 produces a current, from which the concentration of the volatile gas can be determined.
After an extended use of PID 10, contamination, such as a coating of metal atoms, oil film, dust particles, and/or other polymer-like coating, often builds up inside ionization chamber 14, especially on electrodes 20 and 22 and window 16. The contamination on electrodes 20 and 22 reduces the accuracy of volatile gas concentration measurement by preventing the ion and electron collection of electrodes 20 and 22. The contamination on window 16 decreases the intensity of the UV light introduced to ionization chamber 14 from UV lamp 12, and thus reduces the accuracy of volatile gas concentration measurement. Accordingly, PID 10 must be regularly dissembled and cleaned to remove such contamination. A traditional PID usually loses sensitivity to gas by 10% to 20% per day because of contamination during continuous operation.
U.S. Pat. No. 6,225,633, to Sun et al., entitled xe2x80x9cPhoto-ionization Detector For Volatile Gas Measurement and Method for Self-Cleaning The Samexe2x80x9d, issued on May 1, 2001 and assigned to the assignee of the present invention, which is herein incorporated by reference in its entirety, describes a PID that is capable of self-cleaning. After an extended use, the PID is stopped from measuring the volatile gas concentration, and the contamination in the PID is removed without dissembling the PID. When removing the contamination inside the ionization chamber of the PID, the gas sample, which may include the volatile gas molecules, is prevented from flowing through the ionization chamber and thus is trapped within the ionization chamber. Then, the UV light from the UV lamp converts oxygen in the gas sample to ozone, which is a strong oxidant. The ozone etches and removes the contamination inside the ionization chamber, that is, the contamination on electrodes and window. After the contamination has been etched and removed, the products of the contamination removal are discharged from the ionization chamber. After self-cleaning for a period of time, the PID can almost fully recover the sensitivity that was lost due to contamination. But this technique does not offer a continuous operation which is required in most fixed systems for processing control and safety protection.
Although being capable of self-cleaning, the PID of U.S. Pat. No. 6,225,633 has several shortcomings. First, when the gas sample includes little or no oxygen, the effectiveness of the self-cleaning will be decreased. Second, while the PID is being self-cleaned, the PID cannot measure the concentration of volatile gas molecules existing in the ambient area. In other words, the PID cannot perform real-time measurement of the concentration of volatile gas molecules.
During the normal operation of PIDs, the effectiveness of PIDs diminishes due to the build-up of contamination, such as a coating of metal atoms, oil film, dust particles, or other polymer-like coating substances, in an ionization chamber of the PID. The contamination on an optical window of the PID can gradually decrease the UV intensity from a UV lamp to the ionization chamber, resulting in inaccurate measurement of volatile gas concentration. Accordingly, a user must dissemble the PID to clean ionization chamber. Further, while the PID is being cleaned, the volatile gas concentration cannot be measured.
The present invention provides PIDs capable of self-cleaning and real-time measurement of the concentration of volatile gas molecules existing in the environment. The invention also provides methods that enable the PIDs to continuously perform self-cleaning and real-time measurement of the concentration of volatile gas molecules.
In accordance with an aspect of the present invention, a photo-ionization detector (PID) includes a microprocessor and a gas detection unit that measures a current corresponding to a concentration of a volatile gas in an ambient gas. The gas detection unit includes an ionization chamber, a UV lamp that ionizes the ambient gas in the ionization chamber, a bias electrode, and a measurement electrode. The microprocessor controls the gas detection unit such that the flow of the ambient gas in the ionization chamber is intermittently interrupted, and the UV lamp converts oxygen in the closed ambient gas to ozone.
The PID further includes a container that contains an oxygen-containing gas, which is supplied into the ionization chamber when the flow of the ambient gas is interrupted in the ionization chamber, so that the oxygen-containing gas is converted to ozone. The PID further includes a lamp driver, a bias driver circuit, and a measurement driver circuit. The microprocessor controls the lamp driver circuit, the bias driver circuit, and the measurement driver circuit.
The PID further includes a pump and a pump driver circuit, through which the microprocessor controls the pump. The pump is between the microprocessor and the gas detection unit, wherein the microprocessor intermittently closes the flow of the ambient gas in the ionization chamber by turning on and off the pump.
In accordance with another aspect of the present invention, a PID includes a microprocessor and multiple gas detection units. Each of the gas detection units measures a current corresponding to a concentration of a volatile gas in an ambient gas and includes an ionization chamber, a UV lamp, a bias electrode, and a measurement electrode. The microprocessor controls the gas detection units such that the flow of the ambient gas is prevented in the ionization chamber of at least one of the gas detection units while the ambient gas flow through the ionization chamber of at least another one of the gas detection units is permitted. The UV lamp converts oxygen in the closed ambient gas to ozone, which removes contamination in the ionization chamber with the closed ambient gas.
The PID further includes a container that contains an oxygen-containing gas, which is supplied into the ionization chamber, in which the flow of the ambient gas is prevented. The PID further includes a multi-port valve, a pump, and a pump driver circuit, through which the microprocessor controls the pump. The pump is between the microprocessor and the gas detection units, the pump moving the ambient gas through the ionization chambers of the gas detection units. The multi-port valve is coupled to the ionization chamber of each of the gas detection units and the pump, and opens and closes the ionization chamber of each of the plurality of the gas detection units to the flow of the ambient gas, while the pump is turned on.
In accordance with another aspect of the present invention, a PID includes a microprocessor, a first gas detection unit, and a second gas detection unit. The microprocessor controls the first and second gas detection units such that the ambient gas always flows through the ionization chamber of one of the gas detection units and the flow of the ambient gas is prevented in the ionization chamber of the other one of the gas detection units. The UV lamp converts oxygen in the closed ambient gas to ozone, which removes contamination in the ionization chamber with the closed ambient gas.
The PID further includes a container that contains an oxygen-containing gas, which is supplied into the ionization chamber, in which the flow of the ambient gas is prevented. The PID further includes a three-way valve, a pump, and a pump driver circuit, through which the microprocessor controls the pump. The pump is between the microprocessor and the gas detection units, the pump providing the ambient gas through the ionization chambers of the gas detection units. The three-way valve connected to the ionization chamber of each of the gas detection units and the pump permits ambient gas flow in the ionization chamber of one or the other of the gas detection units while the pump is turned on. The flow of ambient gas is permitted in the ionization chamber of one of the first and second gas detection units from which contamination has been removed during a time when ambient gas is prevented from flowing in the ionization chamber of the other of the first and second gas detection units to permit cleaning the ionization chamber of the other detection unit.
Another aspect of the present invention provides a method of real-time self-cleaning and measuring of a volatile gas concentration with a PID that comprises a gas detection unit including an ionization chamber, in which an ambient gas including a volatile gas is ionized by a UV lamp. The method includes causing the ambient gas to flow through the ionization chamber, to permit the PID to measure the volatile gas concentration, and causing the flow of the ambient gas through the ionization chamber and periodically interrupting the flow. The flow is on for a first period of time and off for a second period of time, and during the second period of time the UV lamp converts oxygen contained in the ambient gas to ozone to remove contamination in the ionization chamber. The method further includes supplying an oxygen-containing gas into the ionization chamber.
Flowing the ambient gas and stopping the flow of the ambient gas are repeated. The switch between flowing the ambient gas and stopping the flow of the ambient gas occurs by turning on and off a pump connected to the ionization chamber.
Still another aspect of the present invention provides a method of real-time self-cleaning and measuring of a volatile gas concentration with a PID that includes multiple gas detection units, each of the gas detection units including an ionization chamber, in which an ambient gas including a volatile gas is ionized by a UV lamp. The method includes flowing the ambient gas through the ionization chamber of one of the gas detection units, so that the PID measures the volatile gas concentration and stopping the flow of the ambient gas in the ionization chamber of another of the gas detection units so that the UV lamp converts oxygen contained in the ambient gas in the ionization chamber of the another gas detection unit to ozone, which removes contamination in the ionization chamber with the closed ambient gas.
The method further includes supplying an oxygen-containing gas into the ionization chamber in which the flow ambient gas is stopped.
Each of the gas detection units is repeatedly switched between flowing and stopping the ambient gas. The switch between flowing and stopping the ambient gas is achieved by using a multi-port valve connected between a pump and the ionization chamber of each of the gas detection units.
Still another aspect of the present invention provides a method of real-time self-cleaning and measuring of a volatile gas concentration with a photo-ionization detector (PID) that includes a first gas detection unit and a second gas detection unit, each of the gas detection units including an ionization chamber, in which an ambient gas including a volatile gas is ionized by a UV lamp. The method includes flowing the ambient gas through the ionization chamber of the first gas detection unit, so that the PID measures the volatile gas concentration, and stopping the ambient gas through the ionization chamber of the second gas detection unit so that the ambient gas is closed in the ionization chamber of the second gas detection unit while the ambient gas flows through the ionization chamber of the first gas detection unit. The UV lamp converts oxygen contained in the ambient gas in the ionization chamber of the second gas detection unit to ozone, which removes contamination in the ionization chamber of the second gas detection unit.
The method further includes supplying an oxygen-containing gas into the ionization chamber in which the flow of ambient gas is stopped.
Flowing and stopping the flow of ambient gas are repeatedly switched between the first and second gas detection units. The switch between flowing and stopping the flow of ambient gas is achieved by using a three-way valve connected between a pump and the ionization chamber of each of the gas detection units.